Method of forming an optical device by laser scanning

ABSTRACT

A method of forming an optical device in a body (32), comprises performing a plurality of laser scans (34,36) to form the optical device, each scan comprising relative movement of a laser beam and the body thereby to scan the laser beam along a respective path (34a, 34b 34f; 36a, 36b 36f) through the body to alter the refractive index of material of that path, wherein the paths are arranged to provide in combination a route for propagation of light through the optical device in operation that is larger in a direction substantially perpendicular to the route for propagation of light than any one of the paths individually.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a divisional of U.S. application Ser. No.13/703,463, filed on Mar. 27, 2013, now U.S Pat. No. 10,564,441, whichclaims the benefit of International Application No. PCT/GB2011/000869,filed on Jun. 10, 2011, both entitled: A Method of Forming an OpticalDevice by Laser Scanning, which claims priority to GB 1009810.1 filedJun. 11, 2010, the contents of which are expressly incorporated hereinby reference in their respective entireties.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for formingwaveguide and other optical devices, and to such waveguides and otheroptical devices.

BACKGROUND

Within a fibre optic cable such as that used in optical communications,visible light signals travel along the cable bounded by multiple glasslayers with different refractive index properties; through a processknown as total internal reflection these layers trap the light in thefibre core so it cannot escape until it reaches the end of the cablewhere it can be collected/analysed.

It is well known to include fibre optic cables in an optical sensingnetwork by interfacing them to optical sensors to detect, for example,stress/strain, temperature, humidity, pressure or other properties. Manyof these optical sensors use components called fibre Bragg gratings(FBGs). A Bragg grating is a periodic refractive index variation thatwill reflect light only at a precise colour or wavelength—known as theBragg wavelength. When these structures are present in optical fibresthey are known as FBGs and the precise wavelength of the back reflectedlight will depend on the environment local to the FBG. This concept isnow routinely exploited to create distributed sensing networks for manyapplications.

A key component of such systems is the interrogator. This is the devicethat measures the wavelength shift and produces the calibrated signalthat provides the measure of temperature/stress/strain or other propertyof the system under test. This is where the main cost of a system lies.The interrogator system itself is a precision optical analysercomprising fibre connectors, mirrors, optics and detectors that is bulky(often shoe box sized), expensive to build, can be easily damaged and isa major barrier to mass market uptake.

It is known to form waveguides within bulk material, which operate in asimilar fashion to optical fibres. The waveguides are formed by usingultrashort pulse laser inscription to modify the refractive index of thematerial along a path through the material. One example of a method forforming waveguides in bulk material is described in WO 2008/155548,which is hereby incorporated by reference.

It has also been suggested to form Bragg gratings in such waveguides byvarying the refractive index properties along the path. For example,single scan ultrashort pulse laser inscription using either lowrepetition rate systems where the period of grating is controlled by thesample scan speed or high repetition rate systems where the period iscontrolled using modulation of the pulse train (for example using anacousto-optic modulator) have been suggested as a method of formingwaveguide Bragg gratings.

Examples of methods of forming waveguide Bragg gratings using laserinscription have been described in WO 2007/134438, in which individuallaser modified volumes in a transparent substrate with pre-determineddistances between them in a transmission direction function as bothgratings and waveguide structures. The technique described in WO2007/134438 uses a single laser pass through the material to form thewaveguide and grating structures in a sample, and the grating period iscontrolled by a scan speed of the sample through a focused pulse train.A similar technique is described in an article by Zhang and Herman atwww.photonics.com/Article.aspx?AID=31911.

An alternative approach is described in Marshall et al (Opt.Letters, 31,18, 2690) in which a double pass technique is used. A laser is scannedalong a path through the material once to form a waveguide structure andis scanned along the same path a second time to modify the properties ofthe waveguide to form a grating structure.

However, the control over properties of waveguide or grating devicesfabricated using known techniques of laser modification of bulkmaterials can be limited, and the range of waveguide or gratingstructures that can be produced in practice is also limited. That inturn limits the possibilities of using such techniques for producingpractical sensor systems or for producing interrogators for such sensorsystems.

It is an aim of the present invention to provide an improved or at leastalternative method of forming optical devices.

SUMMARY

In a first, independent aspect of the invention there is provided amethod of forming an optical device in a body, comprising: performing aplurality of laser scans to form the optical device, each scancomprising relative movement of a laser beam and the body thereby toscan a laser beam along a respective path through the body to alter therefractive index of material of that path, wherein the paths arearranged to provide in combination a route for propagation of lightthrough the optical device in operation that is larger in a directionsubstantially perpendicular to the route for propagation of light thanany one of the paths individually.

By using a plurality of scans an optical device, for example awaveguide, of any desired dimensions and properties can be formed in abody. That can provide for improved control over the properties of thedevice, for example improved control over the transverse profile of thedevice and improved mode size control. Improved mode matching to aninput waveguide or fibre and reduced insertion losses can also beprovided.

The improved control over the transverse profile of the waveguide orother device can also provide for the production of a wider range ofdevices, and improved control over their properties in comparison withknown single-scan techniques.

The combination of paths may comprise a single, combined route forpropagation of light, for example a light beam or pulse, through thedevice. The propagation of light through the device may comprisepropagation of light from an input of the device to an output of thedevice.

The paths in combination may provide a predetermined refractive indexprofile for the device. The predetermined refractive index profile maybe a predetermined variation of refractive index with position in adirection substantially perpendicular to the path direction and/ordirection of propagation.

The scanning of the laser beam may comprise causing relative movement ofthe body and the laser beam.

The body may comprise any dielectric material that is at least partiallytransparent to the writing laser beam wavelength including glasses suchas silicates, borosilicates, doped or modified silicates, phosphateglasses, doped or modified phosphates, chalcogenide glasses, doped ormodified chalcogenides, crystalline materials such lithium niobate,yttrium aluminium garnet and also doped, poled or modified crystals suchas periodically poled lithium niobate or Neodymium doped yttriumaluminium garnet, laser or amplifier gain media such as rare earth dopedglasses and crystals.

The light may comprise visible light or non-visible light, for exampleone or more of infra-red light, ultra-violet light or x-rays.

Each path may be offset in a direction substantially perpendicular tothe path direction or propagation direction from at least one other ofthe paths.

For each of the scans, the path scanned by the laser beam may abut or atleast partially overlap at least one of the other paths.

Thus, a desired profile of the optical device can be built up. It hasbeen found that the use of overlapping or abutting regions of scannedmaterial provides a particularly accurate and effective way of buildingup a desired device profile.

Each of the paths that overlap may be a respective region of materialalong which a portion of the laser beam profile having an intensityabove a threshold level is moved during the scan for that path. Thethreshold level may be √2 of the value of the intensity at the focalpoint of the laser beam.

The paths scanned by the laser beam may abut or at least partiallyoverlap at least one other of the paths in a direction substantiallyperpendicular to the path direction and/or the propagation direction.

The method may further comprise selecting the location of each pathand/or selecting at least one property of the laser beam to provide anoptical device having at least one desired property.

The location of each path and/or at least one property of the laser beammay be selected to provide a device having a desired geometry and/orhaving at least one desired optical property. For example, the locationof each path and/or at least one property of the laser beam may beselected to provide a device with a desired transverse profile and/or toprovide a device that is mode matched to a further device or input oroutput source, for example that is mode matched to a waveguide.

The optical device may comprise a waveguide.

For each path, the scanning of the laser beam along the path maycomprise altering the refractive index of material of the path such thatthe path forms part of the waveguide.

The scanning of the laser beam along a path may comprise scanning afocal point of the laser beam along the path. The method may comprise,for each path, focussing the laser beam on the path.

The method may further comprise controlling the laser beam for each ofthe paths to provide a variation of refractive index with position alongthe propagation direction.

Controlling the laser beam may comprise controlling at least oneproperty of laser radiation of the beam, for example at least one ofamplitude and frequency. The radiation may comprise pulsed radiation andthe controlling of the laser beam may comprise controlling at least oneof pulse duration, pulse separation and pulse frequency. The controllingof the laser beam may comprise interrupting, deflecting or focussing ordefocusing the laser beam.

By providing for a variation of refractive index with longitudinalposition along the propagation direction, further types of opticaldevices can be formed instead of or in addition to waveguides. In oneparticularly useful example, the method is used to form a gratingstructure.

The variation in refractive index for the paths may be such as to form agrating structure.

The method may enable the production of high quality waveguide gratingsin laser inscribed devices, in particular ultra-short pulse laserinscribed devices, and such waveguide gratings can be used in turn ascomponents of more complex systems, for example integrated sensors,interrogators, or waveguide lasers.

The use of a multiscan technique to form the grating structure can beparticularly advantageous as it is possible to tailor the tilt andgeneral profile of the grating modulation. This gives a degree offlexibility and functionality that may not be available in previouslyknown direct-write methods. It can also provide for grating structuresof greater quality and for an increased level of control over gratingproperties. For example, in certain embodiments it can provide forcomplete control of grating pattern, arbitrary apodisation (for examplefor pulse shaping or dispersion control), transversely shaped gratings,tilted gratings (for example for sensing or polarisation control) orcurved gratings (for example for control of an output coupledwave-front).

The variation in refractive index may comprise a periodic variation inrefractive index with position.

The variation of refractive index with position may be substantially thesame for one of the paths as for at least one other of the paths.

The variation of refractive index may comprise at least one localmaximum value (and/or local minimum value) of refractive index. Themethod may comprise controlling the laser beam so that at least onelocal maximum value (and/or local minimum value) of refractive index fora path occurs at substantially the same position along the propagationdirection as a corresponding at least one local maximum value (and/orlocal minimum value) of refractive index for at least one other of thepaths.

For at least one path the variation of refractive index with positionalong the propagation direction may be offset, in comparison to thevariation in refractive index with position along the propagationdirection for at least one other path.

The variation of refractive index along each path may comprise at leastone local maximum value (and/or local minimum value) of refractiveindex. The method may comprise controlling the laser beam so that theposition of at least one local maximum value (and/or local minimumvalue) of refractive index for a path is offset in comparison to theposition of a corresponding at least one local maximum value (and/orlocal minimum value) of refractive index for at least one other of thepaths.

By providing for an offset in refractive index with position along thepropagation direction, a wider variety of optical devices can be formed.One particularly useful example is the formation of tilted gratingstructures.

The variation in refractive index may be such as to provide a tiltedgrating structure.

Tilted grating structures can be particularly useful as components ofsystems such as tilted grating sensors, tilted output filters anddistributed beam shaping components.

The use of a multi-scan method allows tilted gratings to be written inbulk samples, for example bulk glass or crystalline samples.

The use of a multiscan method to form waveguides and other devices in abulk sample enables the use of a wide variety of different componentsand the efficient interlinking of such components. In turn that canenable the production of complex optical devices or systems within asingle bulk sample, using a laser scanning technique, and can thusprovide for robust optical devices and systems of small size, forexample having a small footprint. In contrast, corresponding knownoptical devices or systems often require a variety of separatecomponents to be physically attached or aligned with each other,providing for less robust systems or devices, of greater complexity ofconstruction and of greater size.

In a further independent aspect of the invention there is provided amethod of forming an optical system in a body comprising forming aplurality of optical devices in the body, each optical device beingformed using a method as claimed or described herein.

The method may comprise forming the devices such that in operation atleast a portion of one of the devices is coupled to at least one otherof the waveguides.

The system may comprise a plurality of waveguides that, in operation,are evanescently coupled.

The method may comprise forming one of the devices to include a gratingstructure and to form another of the devices to include a furthergrating structure and/or a waveguide, and the devices may be arranged sothat in operation the grating structure is coupled to the furthergrating structure or the waveguide.

The method may comprise forming a grating coupler system.

The method may comprise forming a plurality of cascaded grating couplerdevices.

In another independent aspect of the invention there is provided a laserapparatus for forming an optical device in a sample comprising: a lasersource for providing a laser beam; a sample space for a sample; focusingoptics for focussing the laser beam in the sample space; means forcausing relative movement between the laser beam and the sample space;and a controller for controlling operation of the laser source and themeans for causing relative movement, wherein the controller isconfigured to operate to control the laser source, focusing opticsand/or means for causing relative movement thereby to perform aplurality of laser beam scans, each scan comprising scanning the laserbeam along a respective path through the sample space to alter therefractive index of material of a sample if present in the sample space,wherein the paths are arranged to provide in combination a route forpropagation of light through the optical device in operation that islarger in a direction substantially perpendicular to the route forpropagation of light than any one of the paths individually.

The controller may be configured to control the laser source, focusingoptics and/or means for causing relative movement such that each pathmay be offset in a direction substantially perpendicular to the pathdirection or propagation direction from at least one other of the paths.For each of the scans, the path scanned by the laser beam may abut or atleast partially overlaps at least one of the other paths.

The controller may be configured to select the location of each pathand/or selecting at least one property of the laser beam to provide anoptical device having at least one desired property. The optical devicemay comprise a waveguide.

The controller may be configured to the laser source, focusing opticsand/or means for causing relative movement for each of the paths toprovide a variation of refractive index with position along thepropagation direction.

The variation in refractive index for the paths may be such as to form agrating structure. The variation of refractive index with position maybe substantially the same for one of the paths as for at least one otherof the paths.

For at least one path the variation of refractive index with positionalong the propagation direction may be offset, in comparison to thevariation in refractive index with position along the propagationdirection for at least one other path.

The variation in refractive index may be such as to form a tiltedgrating structure.

In another independent aspect of the invention there is provided atilted grating structure formed in a non-fibre-optic body. The tiltedgrating structure may comprise a waveguide Bragg grating.

In another independent aspect of the invention there is provided agrating coupler system comprising a plurality of grating devices that,in operation, are coupled, wherein the grating coupler system is formedin a non-fibre-optic body.

The tilted grating structure and/or the grating coupler system may beformed by laser inscription of the body. The body may comprise a bulkglass or crystal body.

There may also be provided a method, device or system substantially asherein described with reference to the accompanying drawings.

Any feature in one aspect of the invention may be applied to otheraspects of the invention, in any appropriate combination. For example,device or system features may be applied to method features and viceversa.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the invention are now described, by way of non-limitingexample, and are illustrated in the following figures, in which:

FIG. 1 is a schematic diagram showing laser writing apparatus forforming an optical structure in a bulk sample according to oneembodiment;

FIG. 2 is a flowchart illustrating in overview a method for forming anoptical device;

FIG. 3a is a schematic illustration of a waveguide formed in a bulkglass sample;

FIG. 3b is a diagram showing how the refractive index varies across thewaveguide of FIG. 3a in a direction transverse to the propagationdirection;

FIG. 4 is a schematic diagram of a Bragg grating structure formed in asample using a multiscan technique;

FIG. 5 is a graph of the variation in extinction ratio as a function ofwavelength for the Bragg grating structure of FIG. 4;

FIG. 6 is a schematic diagram of a tilted grating structure formed in abody using a multiscan technique;

FIG. 7 is a schematic diagram of a combination of a grating structureand associated waveguide structures formed in a body;

FIG. 8 is a schematic diagram of another optical device formed in abody;

FIG. 9 is a schematic diagram of a waveguide/grating coupler deviceformed in a body;

FIG. 10 is a graph of the response of FIG. 9 as a function ofwavelength;

FIG. 11 is a schematic diagram of a three-dimensional laser-inscribedwaveguide system; and

FIG. 12 is a schematic diagram of a three-dimensional grating couplersystem.

FIG. 1 shows a laser system for writing apparatus for forming an opticalstructure in a bulk sample. The laser system comprises an IMRA u-JewelD-400 component 6 that has an associated acousto-optic modulator 7 thatcan modulate the pulse train from the IMRA, and is combined with an RFdriver (21080-2AS) that drives the acousto-optic modulator and that isexternally modulated by a signal generator (Keithley 3390). The laserhas an external compressor stage, for emitting a beam of laser radiation8 for forming waveguides in a radiation sensitive material 10, forexample suitable glass or crystal material.

In the embodiment of FIG. 1, the material 10 is carried in a samplespace on a stage structure 12 that can be moved under the control of acomputer based control unit 14. The stage structure 12 comprises aseries of air bearing Aerotech translation stages in three dimensions.(ABL1000 XY and AVL125 vertical). The lateral resolution of the stagesis 2 nm.

In operation, the beam 8 is focused vertically down onto the material 10by a lens system 16, for example comprising an aspheric lens ormicroscope objective having a numerical aperture of 0.4 to 0.67. Thebeam delivery optics are mounted to a granite arch to minimize vibrationand thermal movements. The beam reaches a focal point 18 at a point ofthe material 10 where a waveguide or other optical structure is to beformed.

The high optical power density causes a number of nonlinear opticaleffects in material at the focal region of the beam resulting inpermanent refractive index modification of the material in that region.The substrate can be moved in three dimensions under the focused beam bycomputer controlled movement of the stage structure 12, thus causing atrack of refractive index modification. The pulse train focused insidethe material induces a permanent refractive index modification relatedto a focal volume that is above a threshold level of intensity. Thetranslation of the sample through the focus results in an extrusion ofthe modified volume to create a waveguide. In alternative embodimentsthe substrate remains stationary and the laser beam is moved relative tothe substrate.

Through tailoring of the laser parameters, such as power, polarization,pulse length and speed of translation, structures can be created whichefficiently act as waveguides, or other optical structures, for opticalradiation at a range of wavelengths.

The laser parameters that are used depend on the material properties ofthe sample, and on the desired modifications. In one embodiment, thesample is borosilicate type, Eagle-2000, glass and the writing laser hasa wavelength of between 800-1500 nm, pulse energy of in the region of 10nJ-10 μJ and a pulse width between 100 femtoseconds and 5 picoseconds. Arepetition rate of between 50 kHz-5 MHz is used in this example,although a wider range of repetition rates is available in otherexamples, for example from 100 kHz to 5 MHz. Those laser parametersenable the inscription of a waveguide structure in the sample.

It is an important feature of the embodiment that a waveguide, or otheroptical structure, is produced in the sample by multiple scans of thelaser beam through the material, for example to build up a desiredrefractive index profile. Waveguides can be written in a singletranslation however by building up the waveguide or other opticalstructure in series of slightly offset multiple passes or scans allowsfor significantly more design freedom, allowing the construction of awide range of complex optical systems not accessible using single scantechniques.

In order to construct a waveguide or other optical structure using amultiple scan technique, accurate control of the positioning of thesample relative to the laser beam is needed. In the embodiment of FIG.1, the positional synchronised output (PSO) feature of the Aerotechsystem is used, which provides one, or a series, of electrical triggersignals at a specified position. The trigger signals are received fromstage encoder channels of the Aerotech system which are highly accuratemechanical scales that are used to determine and track changes in axisposition relative to a pre-defined home position of the sampletranslation stages. The trigger signals are used to synchronize thesignal generator and AOM, and provide for accurate, triggered modulationof the laser beam, by way of precise control of position-triggeredsignals from the sample translation stages. An additional interfaceelectronic board is also used to process the output from Aerotech PSOelectronics to remove the effects of spurious spikes which can disruptaccurate triggering. The spurious spikes that are randomly observed onthe PSO trigger signal precede a main gate signal that serves toactivate the external modulation from the AOM driver. The spikes, if notremoved, can confuse AOM triggering such that proper triggering andsubsequent gating of the modulation signal may not be achieved. Theadditional interface electronics board is used to correctly conditionthe signal by latching to an initial incoming rising signal and holdingthe signal high for around 100 microseconds. That buffers the outputsuch that any spurious glitches on the leading edge of the trigger arenot passed to the AOM driver trigger input.

The use of the PSO output allows the relative positioning of the focalregion of the laser beam within the sample between successive scans tobe controlled to an accuracy of around 2 nm using the Aerotech system.

FIG. 2 is a flowchart illustrating in overview a method for forming anoptical device. In the first stage of the method, desired geometricaland/or optical properties of the optical device are selected. Aplurality of different laser beam scan paths and laser beam propertiesto produce the desired geometrical and/or optical properties areselected at the next stage. A body is then positioned in the lasersystem, and a focal region of the laser beam is scanned along theselected scan paths to produce the optical device.

FIG. 3a is a schematic illustration of a waveguide 20 formed in a bulkglass sample 22 of dimensions 2 mm×10 mm×15 mm by ultrafast, for examplefemtosecond, laser inscription using multiple scans. The waveguidecomprises path 24 (delimited by dashed lines in the figure) and path 26(delimited by solid lines in the figure) that overlap. The overlapregion 28 between the paths is indicated by dots in the figure. FIG. 3ais not to scale, the waveguide is significantly smaller in comparison tothe sample 22 than indicated in FIG. 3a , but is expanded in the figurefor clarity.

Each of the paths 24, 26 is formed by scanning of a focal region of thelaser beam along the path during a respective scan. The refractive indexof the material of each path is altered by the laser beam and isdifferent to the refractive index of the material 30 outside the paths24, 26. The difference in refractive index between the material of thepaths and the material outside the paths causes light passing into andpropagating along the waveguide to be confined within the waveguide. Thepropagation of light into and out of the waveguide is indicatedschematically in FIG. 3a by solid arrows. In the embodiment of FIG. 3a ,the path directions and the propagation direction are substantially thesame. In alternative embodiments or modes of operation the pathdirections and the propagation direction of light through a resultingdevice may be different.

The depth at which a waveguide or other device may be formed inside thematerial depends on the working distance of the inscription lens and iscommonly anything from 100 microns to 2000 microns. The width of themultiscan waveguide is generally between 4 and 12 microns depending onthe intended operating wavelength of the device. For example, for awaveguide in Eagle 2000 glass intended to operate at a wavelength of1550 nm the waveguide width and height is around 8 microns. The maximumlength of a waveguide or other device is usually determined by the rangeover which the sample can be translated during the production process,which for the embodiment of FIG. 1 is the range of the translationstages. Samples can be as large or small as required, dependent on thetranslation stage range.

FIG. 3b is a diagram showing how refractive index varies, in a directiontransverse to the propagation direction (and path directions) of afurther waveguide produced in Eagle 2000 glass using a multiscantechnique. In this case, the waveguide is similar to that of FIG. 3a butis built up using around 15 overlapping scan paths. The refractive indexchanges produced by five of the scans individually are represented bydotted lines, and the variation of the refractive index of the waveguidestructure as a whole is represented by a solid line. It can be seen thatthe refractive index of the material of the device of FIG. 3b is alteredfrom a value of 1.477 to a value of 1.480 by the multiple, overlappinglaser inscription processes.

For devices, such as that of FIG. 3b , made of Eagle 2000 glass, therefractive index is usually altered by around 0.2% to 0.3% by themultiscan laser inscription process. The change in refractive indexobtained by the multiscan process depends on the material, and can be ashigh as 5% in chalcogenides or as low as 0.0005%.

In the device of FIG. 3b , the single scan index profiles overlap andproduce a combined change in refractive index. For the Eagle 2000material of FIG. 3b , the refractive index changes produced in a regionof the material saturate after a certain number of scans through thematerial, and so by using a relatively large number of closelyoverlapping scans provides a device with a step-like refractive indexprofile at the edges of the device and a smooth, substantially constantrefractive index profile away from the edges, which is generallydesirable for waveguides and many other devices.

The degree to which the refractive index changes saturate with repeatedscans, and the number of scan required to produce saturation, depends onthe material and the laser parameters used.

For example, the number and positioning of scan paths chosen to producean 8 micron wide waveguide changes with materials and laserparameters—in the case of Eagle 2000 glass, it has been found that 20scans over an 8 micron width can provide for optimisation of propagationloss, and there is no improvement if, for example, 25 scans or 50 scansare used.

In alternative embodiments, each region of material that has itsrefractive index profile altered above a threshold level by a respectivelaser scan may abut, or be separated from, rather than overlap othersuch regions, although the regions may still combine to make a singlepropagation path. The threshold level may be for example √2 of themaximum value of the refractive index change produced by the scan.

By forming a waveguide using multiple scans, as shown schematically inFIGS. 3a and 3b , effective control over, and tailoring of, propertiesof the waveguide can be provided. For example, it is straightforward toselect a desired cross-sectional area of the waveguide by selecting thenumber of abutting or overlapping paths that form the waveguide. Thecross-sectional shape of the waveguide can also be selected by selectingthe relative number of abutting or overlapping paths in each of the twodirection orthogonal to the path directions. The variation in physicalproperties also causes a variation in optical properties, for examplemode size, and the use of a multiple scan technique can provide forimproved mode size control and mode matching, and can reduce insertionlosses. For example, the properties of a waveguide structure can beselected, and built up using multiple scans, to mode match to a singlemode fibre.

It will be understood that the reference to paths is to the pathsfollowed by the refractive-index altering focal region of the laser beamduring the scans. In general, once all of the scans have been performed,each path does not form a separate light propagation path. Instead inoperation light propagates along the waveguide as a whole, made up ofthe combination of paths. In general, if one of the paths were to beconsidered in isolation (for example before the other paths have beenscanned) it would either not support the guidance of a waveguide mode orany such waveguide mode would not have the desired mode profile andwould result in an asymmetric mode or a mode that is the wrong size tocouple with low loss to another waveguide or fibre mode. Waveguides orother devices are made up a combination of the paths (regions ofmaterial that have had their refractive index profile altered by laserscans).

As well as producing waveguide structures, the multi-scan technique canbe used to produce other optical structures and to produce more complexoptical systems including multiple components.

If the pulse train is modulated (using acousto-optic modulation or anyother form of modulation) whilst the sample is translated the opticalstructure is also modulated. For example, using precise modulationtriggered by the sample stages 12, periodic or aperiodic index changescan be built up using multiple scans. Arbitrary apodisation, chirp,phase shifts or cascaded gratings can be fabricated with the appropriatecontrol of the pulse train modulation.

For example, Bragg grating structures can be created within the materialby rapidly modulating the writing beam to produce periodic gratings withfeatures sizes that may be less than 1 micron. The spectral position andshape of the Bragg response can be accurately controlled by altering thetranslation speed and/or modulation frequency. It is also possible toachieve chirped gratings in this way.

Depending on the material a range of writing parameters can be used toform grating structures. For example, in a bulk sample of a borosilicateglass such as Eagle 2000 a writing laser of wavelength between 800-1500nm, a pulse energy of in the region of 10 nJ-10 μJ, a pulse widthbetween 100 fs-5 ps, and repetition rate of between 50 kHz-5 MHz may beused to form a grating structure.

A Bragg grating structure 30 formed in a sample 32 using a multiscantechnique is illustrated schematically in FIG. 4. In this embodiment,one path comprises a series of periodically spaced regions 34 a-34 f inwhich the refractive index is altered by the scanning laser beam, andanother path comprises another series of periodically spaced regions 36a-36 f in which the refractive index is altered by the scanning laserbeam. The spacing of the regions is selected to correspond to a desiredBragg wavelength of the device.

In the embodiment of FIG. 4, the variation of refractive index alongeach path is substantially the same for each of the paths, and thespatially modulated refractive index regions 34 a-34 f and 36 a-36 f areprecisely in-phase.

The multi-scan technique can be used to produce high quality waveguideBragg gratings in bulk material, with desired mode sizes and effectivecontrol over optical properties, compared to gratings produced usingcorresponding single scan techniques. Such laser-inscribed waveguideBragg gratings can provide higher extinction ratios than previouslyreported, for example >30 dB and in some cases up to 40 dB. Thevariation in extinction ratio as a function of wavelength for awaveguide Bragg grating formed in a bulk sample of borosilicate type,Eagle-2000 glass is shown in FIG. 5.

The grating shown in FIG. 4 is a planar grating, which reflects, andtransmits, light in the direction of propagation. The multi-scantechnique can also be used to produce a variety of other gratingstructures having any desired grating profile, including curved,aperiodic or tilted gratings, by suitable selection of path arrangementsand suitable selection of laser parameters for the inscription of eachpath.

A tilted grating structure 40 formed in a body 42 using a multiscantechnique is illustrated schematically in FIG. 6. The grating structure40 is similar to that shown in FIG. 4 and comprises a plurality ofoverlapping or abutting path regions, each path region comprising aseries of periodically spaced regions 44 a-44 f and 46 a-46 f in whichthe refractive index has been altered by the scanning laser beam. Thespacing of the regions is selected to correspond to a desired Braggwavelength of the device.

The grating structure 40 of FIG. 6 is similar to that of the gratingstructure 30 of FIG. 4, but the variation in refractive index for eachof the paths is offset in a direction along the path, in comparison tothe variation in refractive index along another of the paths, in thiscase the adjacent path. Thus, a tilted grating structure is provided.

The tilt of the grating structure can be precisely controlled bycontrolling the offset between scans. The offset can be controlledeither by controlling the synchronisation of the sample movement withthe laser beam modulation (the signal that initiates the modulation isdelayed or advanced by the required amount on each successive scan) orthe sample is translated in x and y whilst the modulation initialisationsignal is sent at specific x (or y) positions regardless of the y (or x)position.

In operation, the tilted grating allows the coupling of a back reflectedsignal out of the structure, for example allowing the signal to bedetected or measured, or coupled into another structure.

In the embodiments of FIGS. 3, 4 and 6 only two paths are shown asmaking up the optical structures, however it will be understood that anysuitable number of paths and any suitable arrangement of the paths inthree dimensions can be selected in order to produce a desired opticalstructure.

For example, a structure could be 8 microns in width, formed using 20scans. An offset of each element in the scan direction can be used tobuild the tilted grating structure, for example if there is an offset of0.4 microns in the propagation direction between each scan a tilt of 45degrees in the plane formed by the index change elements will beprovided.

Grating structures may be formed in combination with waveguidestructures to provide for the guided input and output of light to thegrating structure. A combination of a grating structure 50 andassociated waveguide structures 52, 54 formed in a body 56 isillustrated schematically in FIG. 7, which again is not to scale. Inthis case, the overlapping path regions forming the grating structureare indicated schematically by the dashed lines, and the overlappingpath regions forming the waveguide, non-grating structures 52, 54 areindicated by the solid lines (neither the solid lines nor the dashedlines are shown in FIG. 7 as overlapping, for clarity, although thepaths that they represent overlap).

In the embodiment of FIG. 7, the structures 50, 52, 54 are formed ofthree paths that overlap in a transverse direction. Each of the threepaths is formed in a single scan, with the laser properties (pulseseparation in this example) being changed part way through each scan toform the part of the path forming part of the grating structure 50.

A wide variety of other structures can be formed within a body using themulti-scan laser inscription techniques, and combined to form morecomplex optical structures or devices within a single body. For example,the multiscan technique can be used to produce chirped gratings, gratingcouplers, dispersion control, laser mirrors, pulse compression devices,curved gratings and tilted grating devices.

A further device is illustrated schematically in FIG. 8, which again isnot drawn to scale. In this case two tilted gratings 60, 62 are formedin a body 64 using a multiscan technique. The tilted gratings 60, 62each form part of a respective waveguide structure (not shown) that inoperation guides light to and from the grating structure, and which isformed together with the grating using the multiscan technique.

The tilted gratings are aligned so that in operation light (indicated bysolid arrows in FIG. 8) propagating along one of the waveguides isreflected from the grating 60 into another of the gratings 62 and thenalong the other waveguide structure. Thus, a coupled grating structureis provided. In operation, a portion of the light from one of thewaveguides, at the operating wavelength of the gratings can be sampledfor measurement or detection purposes. Effectively, light at the Braggwavelength is coupled out by the first tilted grating before beingreflected into the adjacent waveguide that includes the second tiltedgrating. The spatial separation of the two waveguides has to be smallenough so that divergence is not too great to prevent effective couplingand also large enough so that the two waveguides are not evanescentlycoupled, for example a minimum separation of 40 microns and a maximumseparation of 100 microns would be suitable for waveguides having indexdifferences of 0.01 or lower.

The multiscan technique that enables accurate control over properties ofthe optical structures can be particularly useful in producing morecomplex, coupled structures such as that illustrated in FIG. 8, as itenables accurate control over the coupling, which is sensitive to theindividual properties and positions of the coupled structures, in thiscase the gratings.

Another device formed using a multiscan technique is illustrated in FIG.9, and comprises a waveguide structure 70 and a further waveguidestructure 72 formed in bulk material using a multiscan laser inscriptiontechnique. The waveguide structures 70, 72 are formed so that thewaveguides run adjacent to each other in a region 74 of the material.The further waveguide structure 72 includes a grating 76, formed duringthe multiscan laser inscription process, at region 74 of the material.The waveguide structures are arranged so that they are sufficientlyclose together at region 74 that in operation they are evanescentlycoupled in region 74.

Evanescently coupled devices can be formed using separation of thewaveguides in the range, for example, 0.5 to 30 microns (separation ofthe closest edges of the waveguide profile). The separation depends onthe index difference in the written waveguide—evanescently coupledwaveguides have been written with a separation of 2 microns with aevanescent coupling region length of 1 mm up to 30 mm—but much greaterseparations and lengths can be fabricated with the same or similarperformance. In the embodiment of FIG. 9, the coupled waveguides inregion 74 are 8 microns wide, single mode at 1550 nm region operationand written in borosilicate glass.

In operation a portion of a light signal input via one of the waveguidestructures 70 is coupled to the grating 76 forming part of the furtherwaveguide structure 72. A part of the light coupled into the grating 76that has a wavelength equal to the operating wavelength of the gratingis reflected by the grating 76 and is output at one end of the furtherwaveguide structure 72, as a reflection signal. The remaining part ofthe light coupled into the grating 76 is transmitted along the furtherwaveguide structure 72 and is output at the other end of the furtherwaveguide structure, and can be referred to as the cross signal. Theremaining input light is transmitted along and exits the waveguidestructure 70, and is referred to as the express signal.

The measured response of the device of FIG. 9 is shown in FIG. 10 whichis a plot of the express, reflection, and straight signals as a functionof wavelength. The straight signal is a signal received from an adjacentstraight waveguide (not shown) that has no grating and is not coupled toany other waveguide, and is shown for comparison purposes. In practice,detectors or measurement devices can be provided to detect or measureany of the signals input to or output from the device of FIG. 9. It willbe understood that the device of FIG. 9 can have a variety ofapplications, for example as a filter, which can filter light at theBragg wavelength of the grating 76, or as a sensor, or as a part of amore complex apparatus. Three dimensional grating coupler devices suchas that shown in FIG. 9 can allow wavelength filtering to be closely andefficiently integrated with detectors/sources, due to the control overmode size and coupling that can be provided by the multiscan technique.

In a further device (not illustrated) two devices of the type shown inFIG. 9 are inscribed in the same device. The cross output of the firstof the devices, for example, can be formed to connect to the signalinput of the second of the devices, thus forming a cascaded gratingcoupler device. Alternatively any other of the inputs or outputs of thetwo devices can be formed to connect together to provide cascadedgrating coupler devices with different geometries. Such cascaded gratingcouplers can be used for a range of sensing or measurement applications.The Bragg wavelengths of the cascaded coupler devices can be differentfrom one another and can be selected for a particular application.

It will be understood that the multiscan laser inscription technique canprovide for the formation of complex, three dimensional optical devices,including any required number and arrangement of waveguide and gratingdevices. Multi-scan grating components can be located anywhere in athree dimensional photonic circuit. Evanescently coupled waveguides canbe inscribed next to the multi-scan grating component to create agrating coupler device whose properties are controlled using the gratingperiod and length.

An example of a further three-dimensional laser-inscribed waveguidesystem is illustrated in FIG. 11, which is not to scale. The system ofFIG. 11 comprises four separate waveguide structures 80, 82, 84, 86 thatare arranged in three dimensions within a single bulk piece of material88. The laser apparatus 90 used to inscribe the waveguides is also shownschematically in FIG. 10.

A further optical system 100 formed in a single piece of bulk material102 is illustrated schematically in FIG. 12. The system comprises anarrangement of seven waveguide structures 104, 106, 108, 110, 112, 114,116 formed in the piece of bulk material 102 using ultrafast multiscanlaser inscription. Each waveguide structure is made up of 20 overlapping(in a direction perpendicular to the propagation direction) paths ofmaterial whose refractive index has been modified by laser treatment.Various pairs of grating structures are formed as part of the waveguidestructures, and are located sufficiently close together to be coupled,and thus to couple pairs of waveguide structures, in operation. Thedirection of propagation of light entering the system is indicated bythe solid arrow 130. The device acts as a wavelength demultiplexer orfilter that can be used to optically interrogate an incoming signal, andcan be used to monitor the wavelength of signals received from a sensornetwork, for example a fibre Bragg grating sensor network.

Optical devices can be formed in a range of different materials using amultiscan laser inscription technique, for example borosilicate glasses,silicates, phosphates, chalcogenides and crystals, also gain media suchas rare earth or transmission metal doped glasses and crystals.

Various embodiments have been described that comprise optical devicesusing grating structures written by multi-scan ultra short pulse laserinscription (MS-ULI). It will be understood that many other devices andsystems can be produced using the described methods. Various devices andsystems can be produced using the described methods that haveapplications in end user markets for sensors (for example inconstruction and other civil engineering, chemical, renewable energy,aerospace or marine engineering, oil/gas, mining, and biotechindustries). For example, the methods can be used in certain embodimentsto produce waveguides and Bragg grating sensor-interrogators on a singlechip to create a low cost, robust sensor interrogator that may, forinstance, be integrated with fibre sensing networks. The gratingcomponent is such a fundamental building block to so many differentdevices that the potential fields of application are almost limitless.

The described methods can, in certain embodiments provide for rapidprototyping of devices and systems, as the high repetition rates thatcan be achieved mean that high scan speeds are possible. The describedmethod can also provide for material design freedom as, with suitablechoice of laser parameters, devices and systems can be formed in a rangeof different materials using the described methods. Furthermore, in manycases no clean room is needed for fabrication of the devices or systems,and fabrication system costs are comparable to those of standard FBGfabrication systems.

It will be understood that the present invention has been describedabove purely by way of example, and modifications of detail can be madewithin the scope of the invention.

Each feature disclosed in the description, and (where appropriate) theclaims and drawings may be provided independently or in any appropriatecombination.

The invention claimed is:
 1. A method of forming an optical device in abody, comprising: performing a plurality of laser scans to form theoptical device, each scan comprising relative movement of a laser beamand the body thereby to scan the laser beam along a respective paththrough the body to alter the refractive index of material of that path,wherein the paths are arranged to provide in combination a route forpropagation of light through the optical device in operation that islarger in a direction substantially perpendicular to the route forpropagation of light than any one of the paths individually; wherein thelaser beam comprises a pulse train and the method further comprisesreceiving a trigger signal from a sample stage for determining arelative positioning of a focal region of the laser beam within the bodyand using the trigger signal from the sample stage to synchronize amodulator for modulating the pulse train whilst the body or laser beamis translated to provide a change in at least one property of the pulsetrain based on position along the propagation direction thereby toprovide a variation of refractive index with position along thepropagation direction.
 2. A method according to claim 1, wherein eachpath is offset in a direction substantially perpendicular to the pathdirection or propagation direction from at least one other of the paths.3. A method according to claim 1, wherein for each of the scans, thepath scanned by the laser beam abuts or at least partially overlaps atleast one of the other paths.
 4. A method according to claim 1, furthercomprising selecting the location of each path and/or selecting at leastone property of the laser beam to provide an optical device having atleast one desired property.
 5. A method according to claim 1, whereinthe optical device comprises a waveguide.
 6. A method according to claim1, further comprising controlling the laser beam for each of the pathsto provide a variation of refractive index with position along thepropagation direction.
 7. A method according to claim 6, wherein thevariation in refractive index for the paths is such as to form a gratingstructure.
 8. A method according to claim 6, wherein the variation ofrefractive index with position is substantially the same for one of thepaths as for at least one other of the paths.
 9. A method according toclaim 6, wherein for at least one path the variation of refractive indexwith position along the propagation direction is offset, in comparisonto the variation in refractive index with position along the propagationdirection for at least one other path.
 10. A method according to claim6, wherein the variation in refractive index is such as to form a tiltedgrating structure.
 11. A method of forming on optical system in a bodycomprising forming a plurality of optical devices in the body, eachoptical device being formed using a method according to claim
 1. 12. Amethod according to claim 11, comprising forming the devices such thatin operation at least a portion of one of the devices is coupled to atleast one waveguide.
 13. A method according to claim 11, wherein thesystem comprises a plurality of waveguides that, in operation, areevanescently coupled.
 14. A method according to claim 11, wherein themethod comprises forming one of the devices to include a gratingstructure and to form another of the devices to include a furthergrating structure and/or a waveguide, and the devices are arranged sothat in operation the grating structure is coupled to the furthergrating structure or the waveguide.
 15. A method according to claim 11,comprising forming a grating coupler system.
 16. A method according toclaim 11, wherein the method comprises forming a plurality of cascadedgrating coupler devices.
 17. A method according to claim 1, wherein theat least one property of the pulse train that changes by the triggeringof the modulator comprises at least one of the pulse amplitude, pulseduration, pulse separation, frequency.
 18. A method according to claim1, further comprising using the trigger signal from the sample stage tosynchronize the modulator thereby to control relative positioning of afocal region of the laser beam within the body between successive scans.